It is well established that SO.sub.2 and NO.sub.x emissions from coal combustion, such as occurring at electrical power plants, promotes acid deposition and the phenomenon known as acid rain. More particularly, the rainfall may be acidified to a pH in the range of 3.5-4.5. This acid rain damages vehicles, buildings and other personal property. It also collects in lakes and streams lowering the pH level of those bodies of water and in some cases adversely effecting those ecosystems. Accordingly, SO.sub.2 and NO.sub.x emissions are a major environmental concern.
Atmospheric fluidized bed combustion (AFBC) is one of a few commercially available technologies presently capable of simultaneously controlling SO.sub.2 and NO.sub.x emissions and maintaining them at acceptable levels when burning relatively high sulfur eastern United States coal. More particularly, SO.sub.2 emissions are limited by capturing sulfur (S) in an appropriate calcium sorbent such as limestone or dolomite. Additionally, NO.sub.x formation is restricted by the lower combustion temperatures inherent to AFBC systems.
Recent AFBC research efforts have focused on further reducing NO.sub.x formation while attempting to maintain high sulfur capture rates and hence reduced SO.sub.2 emissions by utilizing the principals of staged combustion. An example of such an approach is disclosed in U.S. Pat. No. 4,962,711 to Yamouchi et al.
In this patent, a state of the art AFBC apparatus is modified by incorporating a set of tertiary air nozzles in the free board area; that is, the area directly above the fluid or dense phase fluid bed region. Advantageously, due to the reducing conditions provided in the dense phase fluid bed region, NO.sub.x compounds are more efficiently and effectively reduced to N.sub.2 +H.sub.2 O. Accordingly, NO.sub.x emissions are advantageously reduced.
It should be appreciated however, that free board burning is increased with the introduction of the additional air through the tertiary nozzles. As there is less sorbent and fuel contact in the free board area than in the dense phase fluid bed region, the reduction in NO.sub.x emissions is obtained at the expense of decreased sulfur capture. Accordingly, SO.sub.2 emissions increase. In fact, studies have shown that sulfur capture may decrease by up to 30% due to the increased coal burning in the free board area. This leads to a proportional increase in SO.sub.2 emissions.
A further problem with the staged AFBC systems that provide additional over fire air in the free board area relates to the extreme reducing conditions that are then maintained within the fluid bed region. More particularly, these reducing conditions often result in reduced combustion efficiencies leading to an increase in pollutants in the form of incomplete combustion products and also a reduction in power production. Further, calcium sulfide (CaS) is formed during coal firing. CaS is an undesirable reaction product. More particularly, CaS has a propensity to react with water vapor and release hydrogen sulfide (H.sub.2 S). Accordingly, spent sorbent including CaS is not suitable for disposal in a landfill. Consequently, the staged delivery of air into the free board area proposed in the prior art creates further environmental concerns and disposal problems.
While other research has indicated that it is possible to simultaneously lower NO.sub.x and SO.sub.2 emissions with staged AFBC systems wherein additional over fire air is delivered to the free board area, these approaches have all required the utilization of excessively high Ca/S molar ratios. This means that the sorbent is not utilized efficiently or effectively in these systems. Accordingly, these systems require significant additional quantities of sorbent beyond what is desired for economic operation. More specifically, the costs of obtaining and conveying the additional sorbent to the site of the AFBC system and of disposing of the sorbent in an environmentally acceptable manner materially adversely effect the feasibility of commercial operation of this type of system. A need is therefore identified for an improved approach to AFBC.